homologous chromosomes

Homologous chromosomes are a matched pair that carry the same set of genes in the same order — think of them as two editions of the same book, one inherited from the mother and one from the father. They line up alongside each other because they cover the same genetic ground.

Although a pair of homologs carries the same genes at the same positions, they need not carry identical versions of those genes. At any given locus one homolog might hold one allele and the other a different allele; the combination is what produces an organism's genotype. Humans have 23 homologous pairs — 22 pairs of autosomes plus, in females, the two X chromosomes (the X and Y in males are only partly homologous).

Homologous chromosomes play a starring role in meiosis, the cell division that makes eggs and sperm. There they pair up tightly, can swap matching segments through crossing over, and are then separated so each gamete receives just one member of every pair. This pairing and separation is the physical basis of Mendel's idea that the two copies of a gene segregate from one another during reproduction.